I do not understand below usage of using in C++. What's difference with typedef? Could someone explain it with some example?
template<typename DataType> class DataWriter { using ObjType = std::function<void(DataType)> // ... } 
- It's a syntactic sugar brought by C++11. See this post.Igor Korzhanov– Igor Korzhanov2015-01-31 01:00:17 +00:00Commented Jan 31, 2015 at 1:00
- 1In this particular case, it's syntactic sugar. It has other, awesome, uses in addition that typedef doesn't do.Mooing Duck– Mooing Duck2015-01-31 01:02:13 +00:00Commented Jan 31, 2015 at 1:02
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2 Answers
There is no difference in your example to a typedef.
Those are identical:
typedef int a; using a = int; In general, it is more versatile though, which is the reason it was introduced:
It can be templated.
template<class X> using smart = std::unique_ptr<X>;It can be used to import symbols into the current scope.
struct Derived : Base { using Base::Fun; };
3 Comments
Columbo
I don't think the asker wanted an enumeration of all scenarios were the keyword
using appears. He was talking about alias declarations in particular.
Deduplicator
@Columbo: Thanks for the edit. And I don't think the OPs focus is quite that narrow, even if that was the example he gave.
Wood Xia
Thanks all. Firstly, I want to know this "using" usage, secondly I also want to know the reason why C++ 11 introduce this language sugar, I guess there are some reasons.
There is no difference. [dcl.typedef]/2:
A typedef-name can also be introduced by an alias-declaration. The identifier following the
usingkeyword becomes a typedef-name [..]. It has the same semantics as if it were introduced by thetypedefspecifier.
I.e.
using ObjType = std::function<void(DataType)>; Is equivalent to
typedef std::function<void(DataType)> ObjType;